1. Field of the Invention
The present invention generally relates to devices and methods for thermally connecting heatsinks and other heat conductive caps to semiconductor chips.
2. Description of the Related Art
Conventional systems utilize adhesives and thermally conductive pastes to connect cap/integral heatsinks to semiconductor substrate/chip structures. For example, in conventional structures, a layer of thermal paste is placed between the chip and the cap/integral heatsink and the cap/integral heatsink is then joined to the substrate.
However, the height of the chip above the substrate could vary from chip to chip. This is especially true with multi-chip packages. In the case of a paste interface, when cap/integral heatsink dimensional tolerances are included, the thickness of the thermal paste between the top of the chip and the bottom of the cap/integral heatsink could also vary by as much as +/-3 mils.
Since the thickness of the thermal paste in conventional structures could vary by +/-3 mils (and generally exceeds 6 mils in nominal thickness) the heat transfer characteristics of conventional structures vary widely and are less efficient than if the thermal paste gap could be formed more repeatedly and thinner. A discussion of some conventional structures follows.
Some conventional structures utilize a complex cooling hat. Over each of the flip chips on the substrate is a hole in the hat. From each hole extends a spring loaded piston that contacts the back of the chip. These modules are hermetic, and are filled with helium. The primary cooling path is from the circuit side of the chip, through the thickness of the chip, to the face of the piston, up the piston, to the inside of the hat, to the back of the hat, across an interface, to an attached cold plate and to water circulating through the cold plate. The high helium content of the gas in the module greatly reduces the thermal resistances of the chip-to-piston interface and the piston-to-hat interface.
Conventional structures also include a matrix of pistons that contact the back of the chip for cooling. To maintain almost full coverage of the back of the chip, headers are used on the faces of each of the pistons.
Conventional structures also use barrel shaped pistons to allow tighter piston to hat gaps, while maintaining the ability to accommodate chip tilt. Material changes also improve thermal performance. For example, conventionally pistons are made of copper rather than aluminum, and the module can be filled with oil rather than helium.
Further, conventionally solder is included with each of the pistons so that the solder can be reflowed after assembly, to fill the chip-to-piston and piston-to-hat gaps, for improved thermal performance.
In conventional flat plate cooling (FPC), a flat plate just above the array of chips is water cooled. Thermal paste is used to fill the gaps between the chips and the flat plate.
Stable high solids, high thermal conductivity paste is also conventionally available. High thermal conductivity is accomplished by high solids loading, which is accomplished by using a range of particle sizes, and coating the particles with a dispersant. This allows a significant improvement in the thermal conductivity of thermal pastes.
Other conventional structures use spreader plates between the chips and the hat. The spreader plates are soldered to the backs of the chips and then thermally connected to the inside of the hat by a layer of thermal paste.
Conventional structures also include a thermal path that leads first to a cooling plate which has holes that house pistons that will be locked into position. The pistons are joined to the side-walls of the holes by solder, and the piston faces contact the backs of the chips. Conventionally, thermal paste is used to fill the chip-to-piston interface. The assembly conventionally requires two steps, a set up step to reflow the solder and lock the pistons into their final positions (customized to that module) and a separate assembly step where the hat is attached to the substrate, enclosing the chips. The mating surfaces of the hat and piston are metalized for solderability.
The conventional structures use water circulating systems applicable to high-end thermal conduction modules (TCM's), which feature an interface with the external plate which curtails thermal conduction and which is restricted to limited choices of materials, while the design is not readily extendible to air cooled multichip modules (MCM) and low-end MLC modules.